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lll--D 2-.- NASA Technical Memorandum 108997 / ,j 7/::; L_.,j lOP Windshear Radar Calibration, 1992 Flights: Transmitter Power Stability Anne I. Mackenzie August 1993 National Aeronautics and Space Administration Langley Research Center Hampton, Virginia 23665-5225 and Receiver Gain o -4ur_ u_ P,J I -- O" ,4" U cO O- E ,'_ Z :_ 0 Z (.9 C3 <{ E UJ ,,¢C ,, J LLJ U1 U :1: P- UJ U ]:: Pt" Q. 3¢ U. "¢ p- O_ 3= Ul C_ ,--4 C <{ CL') I- .s OZr =El-k-P" I._ _--4 Z CO,

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I. INTRODUCTION In 1992, the Antenna & Microwave Research Branch at the NASA Langley Research Center conducted the second year of experimental Airborne Doppler Windshear Radar flights using Langley's Boeing 737 aircraft, N515NA. This memorandum presents the results of the pre-flight radar calibrations, including transmitter power and receiver system gain and noise. Two series of experimental flights were conducted, one series over Stapleton Airport at Denver, Colorado, and one series over Orlando International Airport at Orlando, Florida. Before each flight, the radar was calibrated using an IFR test set [1], which measured power from the radar transmitter and transmitted echo pulses at pre-set power levels to the radar receiver, allowing receiver gain and noise to be calculated from the recorded signals. The methods of measurement and calculation have been documented in a previous memorandum [2], which describes the radar calibrations for flights conducted in 1991; therefore, only a brief discussion of the methods appears below. All measurements recorded in this document apply to Receiver/Transmitter (R/T) unit #2, which was flown exclusively in 1992. II. QUANTITIES MEASURED OR CALCULATED A. Transmitter Power and Power Amplifier Gain The airborne Windshear Radar was built to transmit pulses at either a low peak power of 200 watts or a high peak power of 2000 watts, nominally. The higher transmitting power was achieved by the use of, effectively, an optional 10-dB amplifier. In pre-flight checks, the transmitter power was measured at both levels with the IFR test set. The actual gain of the 10-dB amplifier was then calculated. Calibrations were performed in radar modes 6 and 7, which are described completely in Appendix A. The primary difference between the two modes was the width of the pulse, mode 6 having a pulse width of 0.96 I_s and mode 7 having a pulse width of 1.92 _s. B. Receiver Gain Except for the changing attenuation provided by the Automatic Gain Control (AGC) system in the receiver, the receiver gain is relatively constant. This constant receiver gain was calculated from I,Q voltages and AGC attenuation values recorded when the IFR input power to the receiver had been -50 dBmW, well above the receiver noise power. The calculation assumed a 50-ohm load resistance seen by the In-phase and Quadrature (I,Q) detectors and did not include the waveguide loss between the antenna and the receiver. Knowledge of the constant receiver gain is useful in that it allows calculation of the received power at the antenna, based on the recorded I,Q voltages and the recorded AGC attenuator settings for a particular range cell, the assumed 50-ohm resistance, the receiver noise power, and the waveguide loss.

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The signal power received at the antenna may be calculated (in linear units) as PAnt = I PLQxAGCG - N ] x LWG (i) where PAnt is power received at the antenna, Pt Q is the I,Q power calculated from the complex I,Q voltage magnitude squared and divided by 50 ohms, AGC is the total attenuation provided by the three AGC attenuators, G is the constant receiver gain, N is the receiver system noise power, and LwG is the waveguide loss (approximately 1.38). Receiver gain was calculated for both the wide bandwidth and narrow bandwidth Intermediate Frequency (IF) filter settings. As shown in figure 3 of [2], the wide IF bandwidth was 7 MHz, while the narrow IF bandwidth was 2 MHz [3]. C. Receiver Noise The equivalent receiver system noise power was calculated as seen at the input to the receiver. The calculations incorporated I,Q voltages and AGC attenuation values recorded when the IFR input power to the receiver had been -127 dBmW, well below the expected receiver noise power. For each IF filter bandwidth, the noise power was found using the average gain calculated at that bandwidth, as described in part B above.

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II1. TABULATED RESULTS OF 1992 CALIBRATIONS There are 10 pre-flight calibration data sets in mode 6. The transmitter power and receiver gain for those data sets are shown below in table 1. Table 1. - TRANSMITTER POWER AND RECEIVER GAIN R/T UNIT#2, Transmitter watts Receiver Gain, dB Power, Flight Date Number Low High Denver 11 7-15-92 178 Denver 12 7-16-92 180 1580 MODE 6, PRE-FLIGHT Wide IF Narrow IF Bandwidth Bandwidth w 123.2 121.7 Denver 13 7-20-92 179 1645 122.8 121.4 1780 123.6 122.3 Denver 15 7-22-92 200 1783 123.1 121.7 Denver 16 7-23-92 182 Orlando 10 8-11-92 184 Orlando 11 8-12-92 186 Orlando 12 8-13-92 188 Orlando 15 8-17-92 190 Orlando 17 8-20-92 196 w 123.0 121.9 _ 123.0 121.8 _ 122.7 121.4 w 123.1 121.9 w 123.4 122.1 Averages 1697 123.1 121.8 186 Data not available Table 2 shows the receiver system noise averaged over all the days when mode 6 measurements were done. Table 2. - AVERAGE RECEIVER NOISE, MODE 6 Wide IF Bandwidth -107.0 dBmW Narrow IF Bandwidth -107.9 dBmW 3

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Four calibration data sets allow calculation of the effective high power amplifier gain. The results are shown in table 3. Table3. - HIGH POWER AMPLIFIER GAIN, MODE 6 Flight Low Power, High Power, Number watts Denver 12 180 Denver 13 179 Denver 15 200 Denver 16 182 Gain, dB watts 1580 9.43 1645 9.63 1780 9.49 1783 9.91 Average 9.62 Four calibration data sets contain same-day measurements done in both mode 6 and mode 7. Table 4 shows a comparison of these mode 6 and mode 7 results, including receiver gain and receiver noise. Table 4. - COMPARISON OF MODE 6 AND MODE 7: RECEIVER GAIN AND RECEIVER NOISE Receiver Gain, dB Receiver Noise, dBmW Flight Mode Number Wide IF Bandwidth Orlando 10 6 123.0 7 123.0 Orlando 12 6 122.7 7 122.7 6 123.1 Orlando 15 7 122.8 6 123.4 Orlando 17 7 123.2 Data not available 4 Narrow IF Wide IF Narrow IF Bandwidth Bandwidth Bandwidth 121.9 -106.8 121.8 -109.1 121.5 -107.2 121.4 -109.6 -110.1 121.9 -106.5 -107.8 121.5 -109.4 -109.8 122.1 -106.7 -107.2 121.6 _

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IV. RECEIVER GAIN FOR R/T #2 DURING YEARS 1991 AND 1992 Figure 1 is a plot of the pre-flight receiver gains for R/T unit #2 over a period including most of the Windshear experiment flights in 1991 and 1992. wide bandwidth narrow bandwidth 124 123 ................................ z <[ 122 Orlando July 121 June Orlando Denver July August // I I I I I I I m m I .... I .... I ,llllaulmHl_t_/li|lolll|lliHt. • • , I .... l , , , , I , , , . I .... 120 160 170 180 190 200 TIME (days in 1991) II 200 210 220 230 240 TIME (days in 1992) Figure 1. - Receiver gain for FIFE#2, calculated from pre-flight calibration data in mode 6 over a 13-month period. The two plots show gains for wide and narrow IF bandwidths. V. RESULT SUMMARY A. Tra,nsmitter Power and Power Am olifier Gain In mode 6, the radar mode most commonly used during the Windshear experiments, the average peak transmitter power was 186 watts, with a standard deviation of 7 watts. In mode 7, the peak transmitter power averaged 7 watts lower than in mode 6. The average effective high power amplifier gain was 9.62 dB, producing an expected 1704 watts peak power in the high power mode. B. R_qeiver Gain The average receiver gain was 123.1 dB for the wide IF bandwidth and 121.8 dB for the narrow IF bandwidth. These results agree with the results from the 1991 calibrations, where the narrow bandwidth produced one dB less gain than the wide bandwidth. The plots in figure 1 show very good gain stability for R/T #2 over the entire course of the Windshear experiments, each gain staying within a range of 0.9 dB. There were no notable differences in receiver gain between mode 6 and mode 7 for a particular IF bandwidth.

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C. Receiver Noise The average receiver noise in mode 6 was -107.0 dBmW using the wide bandwidth IF filter and -107.9 dBmW using the narrow bandwidth IF filter. In mode 7, the receiver noise was lower, the averages being -109.4 dBmW, wide band, and -110.0 dBmW, narrow band. This slightly more than 2-dBmW lower receiver noise for mode 7 is reasonable, since mode 7 used a narrower bandwidth video (Iowpass) filter, corresponding to the wider pulse width. The Iowpass filters are pictured in figure 3 of [2]. 6

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Appendix A: EXPERIMENTAL WlNDSHEAR RADAR MODES Table 5. - Radar Parameters For Modes 6 and 7 Radar Parameter PRF, Hz Pulse Width, I_s Display Mode Display Gain, dB Display Range, nmi Antenna Polarization Scan Angle, deg Tilt Angle, deg Window Delay Time, I_S Range Sampling 2 Range Alias Tape Speed, in/s Antenna Scan Rate, deg/s Number of Range Bins Range Resolution, ft Sampling Interval, ft Window Delay Range, ft Window Length, nmi Alias Windspeed, kn Antenna Scan Mode Antenna Scan Time, s Available Run Time, min 1Weather display mode Mode 6 Mode 7 3755 3755 0.96 1.92 WX 1 WX 32 32 15 15 HOR HOR 0+30 0+30 0+0 0_+0 5.76 7.68 0 0 OFF OFF 30 30 14.625 29.25 91 91 472.1 944.2 472.1 944.2 2833 3777 7.07 14.14 58.8 58.8 AZ AZ 4.1 2.1 60 6O 2 The number of range bins skipped between recorded bins

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REFERENCES [1] IFR Inc., "Operation Manual: RDX/RDC-7708 Weather Radar Test Set," Appendix A: "Specifications,"Wichita, Kansas, 1986, pp. A-1 through A-5. [2] Mackenzie, Anne I., "Windshear Radar Calibration: Transmitter Power and Receiver Gain Stability," NASA TM 107589, June 1992. [3] Schrader, James H., "Summary Description and Operating Characteristics of the Windshear Experimental Radar Data System," NASA CR 189731 (RTI/4500/002-021,Research Triangle Institute, January 1991). 8

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REPORT DOCUMENTATION PAGE Form Approved OMB No 0704-0188 PUOhC eoor_g ouraen for this coitec_lon Ot tnformatlon ,s est,ma: :c 3.e.age " _._r _er "?s_nse. inclu01rtg tl_e [=rne for fevlewtng instructions. _earcr_lng existing data U_C_r cja:heflncJ lna rn&lntalnlng the data nettled. &l co_Dtetlncj ancl rewew_g :he :.lle..=. "f i_fOr_natrOn Sef_o comments rec_arCllng this fden tqma_e Or arW other aspect Ot this Cotlect_ofl ot ir_fofff_llltO, includ_ng Suggelt_o_l$ for fe_ioclr_g th Duraen :C Va_i_r_q_O eaclQuar_r Ser_ce. [)reCl:orate tot Inform&:ion ODera_lon$ 4nd ReDor$. 121S Jeffefso Dav$ HIcJhwiv. Suite 1204. ArhncJton. VA 22202-4302. _nd to ne O,ee of Ma_ager_T ncr Bucige aDerWOrk Recluc'On Pro_ec (0704-018S). Washtngton. DC 20503 1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE Aucjust 1993 4. TITLE AND SUBTITLE i3. REPORT TYPE AND DATES COVERED Technical Memorandum 5. FUNDING NUMBERS Windshear Radar Calibration, 1992 Flights: Transmitter Power and Receiver Gain Stability 16. AUTHOR(S) Anne I. Mackenzie LT. PERFORMING ORGANIZATION NAME(S) AND ADORESS(ES) NASA Langley Research Center Hampton, VA 23681-0001 WU 505-64-12-02 8. PERFORMING ORGANIZATION REPORT NUMBER ANDADDRESS(ES) 10. SPONSORING / MONITORING 9. SPONSORING/MONITORINGAGENCYNAME(S) National Aeronautics and Space Administration Washington, DC 20546-0001 11. SUPPLEMENTARY NOTES IZI. DISTRIBUTION/AVAILABILITYSTATEMENT Unclassified - Unlimited Subject Category 32 13. ABSTRACT (Maximum 200 words) AGENCY REPORT NUMBER NASA TM-108997 12b. DISTRIBUTION CODE During the 1992 NASA airborne Doppler windshear radar flights, radar calibrations were performed prior to each ilight In order to determine transmitter power, receiver gain, and receiver noise power. The calibration results show that the average trotter power In radar mode 6 was 186 watts, with a standard deviation of 7 watts. The average high power amplifier gain was 9.62 dB. At the wlde IF bandwidth setting, the receiver gain was 123.1 dB, while at the narrow IF bandwidth setting, the gain was 121.8 dB. The receiver system noise as seen at the receiver input was -107.0 clBrnw using the wlde IF bandwidth and -107.9 dBmw using the narrow IF bandwidth. In radar mode 7, the receiver gain was the same as in mode 6. However, the receiver noise in mode 7 was about 2.5 cIB less using the wide IF bandwldth and 2.0 ctB less using the narrow IF bandw/dth. The R/T unit flown in 1992 had also been flown the previous year when it produced comparable results. This technical memorandum was written as a follow-up to NASA TM-I07589 (June 1992). which describes similar radar calibrations performed during the 1991 wtndshear radar flight experiments. 14.SUBJECTTERMS 15. NUMBER OF PAGES calibration, receiver system 9 windshear radar, receiver gain noise, transmitter power, airborne 17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION OF REPORT OF THIS PAGE Unclassified Unclassified NISN 7540-0-280-5S00 16. PRICE CODE A02 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT OF ABSTRACT S:anc_arO ;orm 298 (Rev 2-89) 29B-lC,2